fix(spy): record the actual instance in thisValues when spying on a constructor

[saadkhantareen]

Summary

Fixes #1683. When a function or class constructor is spied on and called with new, spy.thisValues[0] did not point to the created instance: it held the throwaway this object created for the proxy invocation, which the constructor never ran on. Properties and methods assigned by the constructor were missing from thisValues[0], and spy.calledOn(instance) did not match the instance the caller received.

Changes

• src/sinon/proxy-invoke.js: in the calledWithNew path, the constructor is invoked through a bound construction, which runs it on a new instance rather than on the this handed to the proxy. When that construction produces an instance of the spied constructor, record it as the call's thisValue for the proxy and its matching fakes, replacing the discarded proxy this. A constructor that returns an unrelated object keeps the original thisValue, since that object was never the constructor's this (this preserves the existing calledWithNew behaviour for such constructors).
• test/src/spy-test.js: two regression tests under .thisValues covering a spied function constructor (the case from Spy's thisValue not pointing to instance when spying constructor #1683, including later mutation of the instance) and a spied ES class constructor.

Tests

• npx mocha test/src/spy-test.js --grep "created instance": 2 passing. Both tests fail on the unmodified code.
• npx mocha test/src/spy-test.js test/src/stub-test.js test/src/mock-test.js: 749 passing, 2 pending, 0 failing.
• npm run test-node: 1561 passing, 12 pending, 1 failing. The single failure is the timing-sensitive Sandbox > warns of potential leak when many fakes are created timeout; unmodified main shows the same single failure (1559 passing), and the test passes in isolation with this change (3 passing).
• npx eslint --max-warnings 0 src/sinon/proxy-invoke.js test/src/spy-test.js: no errors.
• npx prettier --check src/sinon/proxy-invoke.js test/src/spy-test.js: all files use Prettier code style.

Fixes #1683

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